(1) Field of Invention
The invention relates to an amplifier circuit and a method of amplification using automatic gain control.
(2) Description of the Prior Art
Many electrical circuits require a conditioning of a signal to obtain a conditioned signal that satisfies certain predetermined requirements as to e.g. signal level of the signal. For example, a transducer may perform optimally within a certain range of the signal level delivered to the transducer. For this purpose, it is known to use amplifier circuits and amplification methods that receive an input signal, amplify the input signal by a gain factor to obtain an output signal, deliver the output signal to an output terminal, e.g. connected to a transducer, determine a signal level of the output signal and determine the gain factor from at least the signal level. Such amplifier circuits and amplification methods are sometimes referred to as feedback gain control circuits and methods. Also, it is known to use amplifier circuits and amplification methods that receive an input signal, determine a signal level of the input signal, determine a gain factor from at least the signal level, amplify the input signal by a gain factor to obtain an output signal and deliver the output signal to an output terminal, e.g. connected to a transducer. Such amplifier circuits and amplification methods are sometimes referred to as feedforward gain control circuits and methods. For both feedback as well as feedforward gain control circuits and methods, the gain factor may relate the output signal level to the input signal level. It is known to determine the gain automatically in so-called automatic gain control (AGC) circuits and methods. The gain factor may e.g. be determined such that the signal level of the output signal is within a predetermined range, such as between a predetermined minimum target output level defining a lower threshold level and a predetermined maximum target output level defining an upper threshold level. In such case, the output signal may be delivered to the transducer with a signal level that is within the predetermined range, which may e.g. correspond to an optimal operation range of the transducer. The gain factor may also be determined in other ways, e.g., such that the signal level of the output signal is limited to remain below a predetermined maximum level defining a threshold ceiling level. In the latter case, the gain factor may e.g. be a first, constant factor as long as the input signal level is below a certain maximum input signal level, whereas the gain factor may e.g. be substantially inversely proportional to the input signal level when the input signal level is above the certain maximum input signal level, whereby the output signal level would exceed the predetermined maximum level if the gain factor would not be reduced below the first, constant factor. Hereby, the output signal is clipped to the predetermined maximum level, which may e.g. correspond to a maximum operation level of the transducer. Various methods and circuits are known wherein the gain factor may obtain only positive values, only negative values, or values of either sign, and where the gain factor may obtain (absolute) values that remain below zero and one, that remain above one, or that may obtain values ranging from below one to above one. As an example, it is known to use an amplifier circuit arranged to provide an output signal within a predetermined range to a speaker, wherein the output signal is obtained from amplifying an input signal with a gain factor determined in dependence on the signal level of the input signal.
It may be a disadvantage of known amplifier circuits and amplification methods that the amplification may not always be optimal. In particular, it has been found that, for known amplifier circuits and amplification methods, the transducer would have been capable to receive an output signal with a larger output signal level than obtained with the known amplifier circuit and amplification method, i.e. wherein the input signal could have been amplified more strongly.